As keyless entry systems for vehicles, which automatically unlock vehicle doors without mechanical key operation, a smart entry system is proposed. In one smart entry system, a radio communication device (radio transmitter/receiver) mounted in a vehicle starts its communications with an electronic key carried by a vehicle driver only after the vehicle driver manipulates a door from the exterior side. This system drives the doors to the unlock standby condition, when the electronic key is confirmed as the authorized one. However, this system takes some time to enable the vehicle driver to open the door after he touches it.
In another smart entry system, a radio device mounted in a vehicle performs data communications with an electronic key carried by a vehicle driver, when the driver approaches the vehicle. The device then unlocks the doors or drives the doors to the unlock standby condition, if the electronic key is confirmed as the authorized one. In this system, the radio device must continuously or periodically generate radio signals for communications with the electronic key while the vehicle is held parked, because it is impossible to estimate when the vehicle driver having the electronic key returns to the vehicle. Further, this device must be mounted in each door in the vehicle, so that the generated radio signal may be received by the electronic key whichever directions the vehicle driver is. This system thus consumes too much electric power of a storage battery of the vehicle.